On 15/02/2023 15:27, Richard Earnshaw wrote:
>
>
> On 15/02/2023 15:10, Jonny Grant wrote:
>> Thank you for your quick reply Richard.
>>
>> On 15/02/2023 14:30, Richard Earnshaw wrote:
>>>
>>>
>>> On 15/02/2023 14:18, Jonny Grant wrote:
>>>> Hi
>>>> Has GCC considered an improvement to "compile out" from the builtin printf the strings? That being to change it to just be something like puts("file /app/example.cpp:4")
>>>> I had a look, but couldn't find it being asked before.
>>>>
>>>> This is just a short example to demonstrate.
>>>> It would be useful to see the exact string in the debugger "file /app/example.cpp:4", also it saves a few lines of asm.
>>>>
>>>> https://godbolt.org/z/aKz3o6aPd
>>>>
>>>>
>>>> int main()
>>>> {
>>>> __builtin_printf("file %s:%d", __FILE__, __LINE__);
>>>> }
>>>>
>>>>
>>>> .LC0:
>>>> .string "/app/example.cpp"
>>>> .LC1:
>>>> .string "file %s:%d"
>>>> main:
>>>> subq $8, %rsp
>>>> movl $4, %edx
>>>> movl $.LC0, %esi
>>>> xorl %eax, %eax
>>>> movl $.LC1, %edi
>>>> call printf
>>>> xorl %eax, %eax
>>>> addq $8, %rsp
>>>> ret
>>>
>>> We already do when the printf contains simply the format string and no additional arguments.
>>>
>>> I guess it might be possible to handle cases where all the arguments are constant, but even that has its problems, eg:
>>>
>>> - can we guarantee identical output to the platform printf?
>>
>> That's a good question. I was using __builtin_printf so that should be GCC I expect.
>>
>
> No, __builtin_printf is really just an internal hook that is used to handle optimisations of printf - if you look at your assembly output you'll see it is translated back to printf for the C library to handle.
ok I see. I changed the example to take off the end "\n" and it goes back to printf (otherwise it is puts() )
https://godbolt.org/z/W46esaWKd
>
>>> - does it cause string bloat (what if there were 30 or so such statements in your program all identical except for the line number)?
>>
>> That's probably what I am expecting, to see those 30 different formatted strings.
>>
>>> - does it even happen often enough to be worth adding (and maintaining) support? Nothing comes for free in a compiler and the optimisations have to be worth-while in the real world.
>>>
>>> R.
>>
>> You're completely right, it could bloat the file with strings.
>>
>> I can do some with multi-line literals, to get "file /app/example.cpp compiled Feb 15 2023"
>> __FILE__ and __DATE__ worked ok.
>>
>> but it didn't like me putting __PRETTY_FUNCTION__ in the middle. Maybe I'm missing something obvious. Likewise I can't use __builtin_LINE() as that is a function rather than a string. Maybe __PRETTY_FUNCTION__ and __FUNCTION__ are calls to __builtin_FUNCTION().
>>
>> int main()
>> {
>> const char * s = "file " \
>> __FILE__ \
>> " compiled " \
>> __DATE__ \
>> "\n";
>>
>> __builtin_printf("%s", s);
>> __builtin_printf("%s\n", __PRETTY_FUNCTION__); // didn't work when I put in the middle.
>>
>
> That's because __func__, __FUNCTION__ and __PRETTY_FUNCTION__ are not string literals but implicitly declared identifies containing a constant string literal as the initializer - you can't therefore combine them with string literals by simple concatenation. See
>
> https://gcc.gnu.org/onlinedocs/gcc/Function-Names.html
>
> which clearly states this.
>
Ok, thank you for the link, I understand it wouldn't support simple concatenation of __func__ etc the way __DATE__ works.
I remembered I could convert macro arguments in into a string constant. So could get the __LINE__ which is enough with the file name into one string with GCC.
#define xto_str(s) to_str(s)
#define to_str(s) #s
int main()
{
const char * s = __FILE__ ":" xto_str(__LINE__);
__builtin_printf(s);
}
.LC0:
.string "/app/example.cpp:6"
main:
subq $8, %rsp
movl $.LC0, %edi
xorl %eax, %eax
call printf
xorl %eax, %eax
addq $8, %rsp
ret
